Though interest in the emotion of gratitude has historically focused on its role in social exchange, new evidence suggests a different and more important role for gratitude in social life. The find-remind-and-bind theory of gratitude posits that the positive emotion of gratitude serves the evolutionary function of strengthening a relationship with a responsive interaction partner (Algoe, Haidt, & Gable, 2008). The current article identifies prior, economic models of gratitude, elaborates on unique features of the find-remind-and-bind theory, reviews the accumulating evidence for gratitude in social life in light of this novel perspective, and discusses how the find-remind-and-bind theory is relevant to methodology and hypothesis testing. In sum, within the context of reciprocally-altruistic relationships, gratitude signals communal relationship norms and may be an evolved mechanism to fuel upward spirals of mutually responsive behaviors between recipient and benefactor. In this way, gratitude is important for forming and maintaining the most important relationships of our lives, those with the people we interact with every day.